Due to unavoidable process tolerances, continuous-time integrated filters must be tuned, because their pole-frequencies are set by ratios of two technologically independent parameters: transconductance g.sub.m in g.sub.m -C filters, conductance g.sub.ds in MOSFET-C filters, or resistance R in active R-C filters, and capacitance C. Process variations cause g.sub.m /C, g.sub.ds /C, and 1/(RC) ratios to change up to 30-50%. Additionally, the temperature variation may cause excessive variation of g.sub.m or g.sub.ds up to and above 100% depending on the temperature operating range, the technology, and the design of a particular filter. Similarly, phase shifts of amplifiers, that filter integrators are built with, must be compensated even for moderate values of quality-factor (Q), if the filter specifications are to be correctly realized. This compensation becomes particularly important in high-Q filters, where the effects of phase shifts for a transfer function are very detrimental and may lead to instability of a filter.
In many practical applications it is desirable that both frequency- and Q-tuning are performed when the filter is in operation, with the main signal applied. As a consequence, Master-Slave tuning schemes have been implemented, in which errors in frequency and Q of Master designed as a partial replica of the main filter (Slave) are measured and corrections are applied to both the Master and Slave. The pole-frequencies and Q's of Slave filter sections is controlled by Master filter or by Master oscillator, both referred to an accurate frequency and amplitude reference. Total filter accuracy is maintained by continuously applying the frequency reference. The main disadvantage of Master-Slave method is the fact that errors are measured in a filter that is different from the tuned one. The accuracy of Master-Slave method is only as good as the matching between Master and Slave limited by on-chip component matching and, especially, by simplifications of Master structure with respect to Slave.
A need has thus arisen for a filter that provides for direct and accurate on-line tuning, and that does not depend on the accuracy of the matching between Master and Slave.